1. Field of Invention
The present invention relates to prosthetic implant systems, specifically replacement of the articulating surface of the pelvis (the acetabulum). The present invention relates to the use of modular preformed bone fixation mantles used to increase the versatility of existing acetabular replacements and to increase the durability of bone fixation of prosthetic acetabular cups to the skeleton.
2. Description of the Related Art
Total hip replacement is performed by replacing the worn surfaces of the hip joint with mechanical components fabricated from metal and plastic. The socket component (i.e. the acetabular cup) consists of a hemispherical shell, commonly manufactured from ultra high molecular weight polyethylene (UHMWPE) which can be directly fixed within the acetabulum of the pelvis with acrylic cement. Alternatively, the polymeric cup may be mechanically fixed to the reamed acetabulum by means of an intermediate metal shell to which the cup is fixed in surgery. This form of fixation is referred to as the "cementless mode". To render it stable within the pelvis under the action of joint loading, the metal shell generally has a porous outer surface into which bone and soft tissue can grow. At surgery, the shell is initially fixed to the bone, and the liner is then fixed in place within the shell. Typically, mechanical features within the inner surface of the shell and on the outer surface of the liner allow the surgeon to fix the liner in a desired rotational position within the shell. This allows the surgeon to maximize the mechanical stability of the prosthetic joint once the socket and the prosthetic femoral component are mated to form the artificial joint.
Cemented, single piece acetabular prostheses are fixed in place with acrylic cement which is introduced in a doughy or liquid form. Once the cement polymerizes, the outer surface of the shell becomes rigidly attached to the reamed surface of the acetabulum. Typically, the cement is delivered to the implantation site by hand or via a syringe. Once the acetabulum is filled with cement, the prosthetic component is pressed into the bed of cement until the surgeon determines that a satisfactory position has been obtained. At this point, excess cement which has extruded from the space between the bony socket and the prosthesis is removed and the prosthesis is held in position until the acrylic cement polymerizes. This may take from five to ten minutes.
Cement fixation has two major disadvantages. First, the strength of attachment of the cement to the bone depends upon the ability of the cement to penetrate the narrow, bloody interstices of the reamed bony bed of cancellous bone. To achieve this end, the cement must be forced into the bone under pressure. The cement, however, is difficult to pressurize with existing devices used for cement pressurization, thus many surgeons use the act of implantation to effect pressurization. Unfortunately, even this latter method is unsatisfactory because the cement pressure only becomes large enough to cause penetration of cement into the porous bone over the last few millimeters of implantation (i.e. when the peripheral gap has closed to less than about 1 mm). Cement mantles of this thickness (i.e. about 1 mm) are prone to fragment under repetitive weight-bearing, leading, ultimately, to loosening of the prosthesis. A second disadvantage is that during the process of driving the prosthesis into the acetabulum, the surgeon is not able to control its precise position with respect to the bony socket. Consequently, the polymeric surface of the prosthesis often contacts the bone in discrete areas, leaving areas with little or no cement mantle which can, once again, lead to fragmentation and loosening. Further, as soon as any part of the prosthesis contacts the walls or rim of the socket, the pressurizing force passes directly to the bone and pressurization is lost.
Previous attempts to solve the foregoing problems have included a variety of methods to pressurize cement within the acetabulum. Commonly proposed methods include the use of hemispherical compaction devices prior to implantation of the prosthesis. Others have developed special deformable or inflatable seals that are placed around the nozzle of the cement syringe and are forced into the implantation site. These devices attempt to fit the mouth of the acetabulum to allow the liquid cement to be injected under pressure without leakage. In general, these methods are unpopular and are relatively difficult to use effectively.
Previous attempts to improve fixation of joint prostheses to the cement include the use of femoral prostheses coated with a thin layer of polymerized cement, such as polymethylmethacrylate (PMMA). When the implant is inserted into the femur, the bone cement reacts chemically with the coating to provide a stronger adhesive bond between the implant and the cement mantle. These systems, such as those described in U.S. Pat. No. 4,336,618 to Raab and U.S. Pat. No. 4,491,987 to Park, for example, require relatively long, and sometimes complicated, procedures for preparing the cement-coated implant. While precoating is performed to increase the strength of the adhesive bond formed between the implant and the cement mantle, it is believed that it has no effect upon the strength of the cement itself or the bond formed between the cement mantle and bone. Moreover, it plays no role in controlling the alignment of the prosthesis or in controlling the thickness of the cement layer.
The cementless method of fixation of acetabular prostheses eliminates the use of acrylic cement and relies upon purely mechanical attachment of the prosthesis to the pelvis. Most "cementless" acetabular components consist of a metal shell and a polymeric liner. Typically, the metal shell is fixed to the acetabulum without cement, using instead an interference fit, metal screws, or both. Most shells have a rough porous outer (convex) surface which facilitates the development of friction between the shell and the bone during implantation. Typically, a shell is selected which is slightly larger (i.e. 1-3 mm) than the diameter of the reamed acetabulum. The shell is manually implanted into the bone. During this process, the surgeon attempts to control the position of the shell, particularly its angular orientation with respect to the bone and its depth of implantation. Once the shell is implanted, the surgeon may supplement its fixation by passing one or more screws through holes within the shell, thereby compressing the shell into the bone and increasing the stability of the interface between the shell and the socket.
The acetabular prosthesis is assembled by placing the polymeric liner into the hemispherical shell and locking it in place by impaction. Prior to assembly of the prosthesis, the preferred orientation of the liner within the shell is ascertained using a facsimile of the liner which does not engage the locking features of the shell. During this procedure, the femoral prosthesis is mated with the facsimile, and the artificial joint is moved to its extremes of motion to determine whether it will provide adequate excursion without dislocation or instability. In the event that insufficient motion is present, the surgeon is able to rotate the liner within the shell or replace the liner with another of a different design in order to increase the amount of stable motion. The ability of the surgeon to revise the position of the liner within the shell is one of the advantages of the two-piece modular construct. Once the surgeon has selected the best combination of design and orientation of the liner, the acetabular prosthesis is impacted, and the implantation is complete.
Cementless acetabular prostheses have a number of disadvantages, however. The initial stability of attachment of the prosthesis to the pelvis is not as rigid as those provided by fixation with acrylic cement. This can lead to pain and even dislodgement of the shell from the acetabulum. With time, tissue grows into the surface of the metal shell, but only a small proportion of the available surface is directly bonded to bone, leaving the rest of the interface susceptible to infiltration by small particles generated through wear of the polymeric articulating surface. Though cemented cups also have disadvantages, the bone cement at least provides an effective method of sealing off the interface with the underlying bone, thereby slowing or limiting the intrusion of wear particles.
Cementless acetabular prostheses also increase the cost of the joint replacement procedure. This is primarily due to two factors that contribute to much of the cost of manufacture: (i) the metal shell generally has a porous outer surface to promote ingrowth of bone, and (ii) the inner (concave) surface of the shell must be precisely machined and toleranced to ensure that the polymeric liner locks into place with minimal relative motion between the liner and the shell during subsequent loading within the body.
U.S. Pat. No. 4,955,325 to Zarnowski, et al. describes a method of converting a cementless acetabular cup component to one suitable for cemented affixation by means of attaching a plurality of spacers to the outside surface of the cup. While this theoretically allows a uniform layer of bone cement to form between the shell and the acetabulum and may reduce the incidence of direct impingement of the prosthesis on the underlying bone, the cement layer is not applied under pressure, and thus many of the same problems associated with cemented prostheses are still present. Moreover, the spacers are discontinuous and do not seal the implantation site during implantation of the prosthesis. Consequently, the spacers do not enhance the pressurization of the liquid cement to any significant degree.
It is therefore desireable to have a prosthetic system that combines the benefits of cemented and cementless acetabular prostheses. In particular, it is desireable to have a prosthetic system that has many of the following attributes:
(1) controls the thickness of the mantle; PA1 (2) prevents direct impingement of the prosthesis on the underlying bone; PA1 (3) allows the bone cement, when used, to be pressurized during implantation of the prosthetic acetabular shell without compromising the thickness of the mantle; PA1 (4) allows the rotational position of the polymeric articulating surface to be altered after testing the range of motion of the joint without risk of damage to the acetabulum; PA1 (5) allows a metal prosthetic acetabular shell designed primarily for use without cement to be utilized with cement or bone fixation enhancing substance without any modification prior to surgery; and PA1 (6) is cost effective by minimizing the need for a large number of different sizes of costly metal and/or polymeric prostheses.